1. Field of the Invention
The present invention generally concerns the structure of a magnetic actuator, one of a number of such radially arrayed in a wire (dot) matrix printhead, which actuator is improved for better packaging density and better coupling of magnetic forces. Specifically, the present invention concerns a three-pole magnetic actuator (1) with a single coil wound around the center magnetic pole, (2) with an "E"-shape core, and (3) with an armature striding over all three magnetic poles.
2. Description of the Prior Art
A wire, or dot, matrix printer includes a number of electromagnetic actuators for use as drive elements of the dot matrix. Printhead designs in the prior art generally consist of a series of such electromagnetic actuators arranged in a radial, or circular, array, or within a succession of superimposed such radial arrays. The actuators contain a yoke, generally of two poles, with armatures striding across the two poles. The armature connects to a force-transmitting wire, and acts as a lever to drive such wire in order to make a printed dot upon the workpiece paper.
One major electromagnetic actuator design present in the prior art employs two poles. This two-pole design is present in two major varients: one variant in which the armature pivots at the outer pole of the yoke, and another variant wherein the armature pivots at the inner pole of the yoke. Since magnetic flux flows in a symmetric way, such designs of a two-pole yoke carry only a fraction of the flux, "throwing away" almost 50% of the useful flux into the air. Further, such a design has undesirable width, or thickness, in the direction along which a number of such magnetic actuators will be radially arrayed to form a printhead actuating a like number of wires in order to form a like number of dots constituting a printed character. Such prior art two-pole magnetic actuators for wire matrix printers are distinquishable from the three poles of the present invention of a magnetic actuator, which invention is also of a specific "E"-shaped core geometry supporting tight packing.
Another class of prior art electromagnetic actuators for use as dot-matrix printer drive elements employs the magnetic "core and shell" geometry. This geometry is particularly visible in the figures of U.S. Pat. No. 3,828,908 for MOSAIC PRINTHEAD to Schneider. Each of the actuators consists of (1) a magnetic core and shell, (2) an armature, and (3) a bobbin where the armature is attached. Further use of "core and shell" magnetic actuators is shown in U.S. Pat. No. 3,876,050 for ARMATURE STRUCTURE FOR MOSAIC TYPE PRINTER to Linder, U.S. Pat. No. 4,236,836 for DOT IMPACT PRINTER AND ACTUATOR THEREFOR to Hodne, and U.S. Pat. No. 4,279,521 for WIRE MATRIX PRINT HEAD to Kightlinger. The Hodne patent particularly shows detailed illustration of the actuators, which have a cross-section of cylindrical geometry. Although such electromagnetic actuators might conceivably be said to embrace three-poles, and so appear when taken in cross-section, they are considerably different from the two-dimensional design geometry of the present invention based on a substantially two-dimensional "E"-shaped core. The cylindrical magnetic "core and shell" geometry poses a severe limitation on mechanical packaging. A considerably larger size along the direction of the array of such electromagnetic actuators is present than is present in the present invention of essentially two-dimensional three-pole yoke geometry. An interesting prior art solution to this packaging problem is shown in the Linder and Kightlinger patents wherein superimposed arrays of electromagnetic actuators are employed because the total number of actuators required cannot be circumferentially arrayed in a single circle of desired circumference. This is due to the physical dimension, circumferentially along such circular array, of each of such electromagnetic actuators.